1. Field of the Invention
The present invention relates to an optical pickup for achieving recording/reproduction of information in such an information recording medium as DVD and for detecting prepits which are preliminarily recorded on the information recording medium and which indicate various control information such as address information.
2. Description of the Related Art
Recently, in an information recording medium in which information can be additionally written, such as DVD-R (DVD-Recordable), a plurality of prepits indicating various control information such as address information are preliminarily formed on the information recording medium at preformat stage. When recording or reproduction is carried out with respect to the information recording medium, necessary control information is obtained by detecting the prepits. The prepits are generally formed as phase pits in a land track of the information recording medium. The detection of the prepits is carried out with a light receiving device installed in an optical pickup which is usually used for reproduction of information pits. That is, both the detection of the prepits and the detection of the information pits are simultaneously carried out with the single light receiving device.
To realize a focusing servo control, astigmatic method is used. The astigmatic method is a method of detecting a focus error by positively using astigmatism. To produce astigmatism in a light beam, a multilens or a cylindrical lens is used as the optical system in the optical pickup. In such an optical pickup, the light beam is passed through the multilens or the cylindrical lens and then is entered the light receiving device. The light receiving device thus generates a detection signal corresponding to the received light beam. This detection signal is utilized for generation of an RF (Radio Frequency) signal, detection of a tracking error, detection of the prepits, and other various operations as well as detection of the focus error.
However, producing astigmatism in the light beam causes deflection of the light beam. This causes the following problem. In a typical optical pickup, the light receiving device has a light receiving portion that is divided into a plurality of regions, for example, four regions. The optical pickup simultaneously receives the light beam with the respective divided regions, generates a push-pull signal, and outputs this signal as the detection signal. If the deflection occurs in the light beam by producing astigmatism, a light receiving pattern is deflected to one way. This causes the inaccuracy of the detection of the prepits. Further, if astigmatism produced by the multilens mixes with astigmatism produced by other optical systems, various undesirable phenomena occur in the light receiving pattern, for example, rotation of the light receiving pattern, appearance of oblique astigmatism.
lt is therefore an object of the present invention to provide an optical pickup which can accurately detects prepits with a simple structure, using an optical device for producing astigmatism in a light beam.
An optical pickup in accordance with the present invention is a device for reading information from a recording medium having a plurality of prepits or a device for recording information onto the recording medium. The optical pickup includes: a light emitting device for emitting a light beam to the recording medium; a dividing device for dividing the light beam reflected by the recording medium into a first divided beam and a second divided beam; an astigmatism producing device for producing astigmatism in the first divided beam; a first light receiving device for receiving the first divided beam from the astigmatism producing device; and a second light receiving device for receiving the second divided beam from the dividing device.
In the optical pickup according to the present invention, the light beam reflected by the recording medium is divided into two light beams, namely, the first divided beam and the second divided beam. The light beam reflected by the recording medium contains various information, for example, information of prepits and information to be used for focus error detection. Both the first divided beam and the second divided beam contain these information. The first divided beam is supplied to the astigmatism producing device. The astigmatism producing device produces astigmatism in the first divided beam. The first divided beam is then supplied from the astigmatism producing device to the first light receiving device.
On the other hand, the second divided beam is supplied to the second light receiving device. Because the second divided beam is not supplied to the astigmatism producing device, the astigmatism is not produced in the second divided beam by the astigmatism producing device.
The first divided beam received by the first light receiving device is used for focus error detection for example. As the astigmatism has been produced in the first divided beam, the focus error detection can be appropriately carried out.
On the other hand, the second divided beam received by the second light receiving device is used for prepit detection. Because the astigmatism is not produced in the second divided beam by the astigmatism producing device, deflection of the light receiving pattern of the second divided beam on the second light receiving device can be reduced or removed. Therefore, the accuracy of the prepit detection can be increased.